I use MoI at the moment for some steel construction thing. giving the supplier the outlines (boxes and cylinders) from Moi and receiving the finished models in .stp format exported from Inventor as I've heard today.

the .stp file is ~2.5mb in size. When imported and saved as .3dm the file size goes to 55mb; exported to .stp it is even more so I wonder why the differences in file size are so big... maybe the file saving routines are so different - but is this something you might want to tackle anytime in future?

It could be possible that the STEP file is set up with several parts repeated in it using something like a block definition and then instances of that. That kind of instancing system helps to reduce file size quite a bit since all the instances share the same full object data.

MoI does not currently have an instancing system like that, so probably when that gets imported into MoI everything basically expands into just regular copied geometry.

In the future I do want to add an instancing system to MoI.

But I'm not totally sure that is the particular issue that you're seeing or not, without seeing any actual files I can only guess.